A microphone is a transducer that converts patterns of air pressure (i.e., an acoustic signal) into an electrical signal. In a typical dynamic microphone, a microphone diaphragm moves a coil relative to a magnetic field in order to cause current to flow within the coil. In a typical condenser microphone, a microphone diaphragm (e.g., a charged metallic plate, an electret, etc.) moves relative to a rigid backplate in order to cause current to flow from a power supply attempting to maintain a constant potential difference between the microphone diaphragm and the rigid backplate.
Wind noise can interfere with a microphone's ability to sense an acoustic signal. For example, when a person speaks into a microphone, wind noise can mask out the person's voice thus obscuring the person's voice from a device attached to the microphone (e.g., an amplifier, a recorder, a transmitter, a speaker, etc.). Wind noise can also mask out vital acoustic information reducing the performance of automated systems such as automatic object/target recognition devices, direction finding systems, etc.
Some microphone assemblies include windscreens that cover microphones in order to reduce wind noise sensed by the microphones. One conventional windscreen, which is typically seen on top of a microphone held by a television reporter, is made of foam and has a spherical shape (e.g., a foam ball which is approximately 10 centimeters in diameter covering the microphone). Such windscreens have been used for many years and can be effective in suppressing wind noise (e.g., an annoying rumbling sound) that could otherwise obscure particular sounds of interest (e.g., the television reporter's voice).
Some scientific experiments have attempted to electronically remove wind noise from sound and wind noise at a target location (e.g., to obtain an acoustic signature from a passing truck). In general, these experiments used a microphone for sensing sound and wind pressure, a set of hot-wire anemometers disposed around the microphone (e.g., a few millimeters from the microphone) for sensing wind velocity, and computerized equipment for storing and processing the sound and wind pressure sensed by the microphone and the wind velocity sensed by the set of hot-wire anemometers. A typical hot-wire anemometer is a fragile device that senses wind velocity by heating a short piece of wire (e.g., a 1.5 mm length of tungsten or platinum), and measuring the heat lost due to wind blowing past the wire (the heat or energy loss being directly related to the wind velocity).
One of the above-mentioned experiments occurred as follows. A first analog-to-digital (A/D) converter converted a signal from the microphone into a digitized sound and wind pressure signal which was stored in the memory of a computer. Simultaneously, a second A/D converter converted a signal from the set of hot-wire anemometers into a digitized heat-loss signal which was also stored in the memory. Next, a digital signal processor processed the sound and wind pressure signal and the heat-loss signal. In particular, an algorithm was applied to the heat-loss signal to generate wind pressure data, and the wind pressure data was subtracted from the sound and wind signal. Although the experiment provided mixed results, in theory the end result should have been a sound signal from the target location with wind noise removed.
An experiment along the lines mentioned above is described in an article entitled “Electronic Removal of Outdoor Microphone Wind Noise,” by Shust et al., Acoustical Society of America 136th Meeting Lay Language Papers, October, 1998, the teachings of which are hereby incorporated by reference in their entirety. Another experiment along similar lines is described in an article entitled “Low Flow-Noise Microphone for Active Noise Control Applications,” by McGuinn et al., AIAA Journal, Vol. 35, No. 1, January, 1997, the teachings of which are hereby incorporated by reference in their entirety. Such experiments provided some encouraging test results, but only when the wind flow was substantially normal incident to the microphone diaphragm. A related experiment and wind signal algorithms (e.g., fluid dynamic equations) are described in a dissertation entitled “Active Removal of Wind Noise from Outdoor Microphones using Local Velocity Measurements,” by Shust, Ph.D. Dissertation in Electrical Engineering, Michigan Technological University, Mar. 6, 1998, the teachings of which are hereby incorporated by reference in their entirety.